Now we look at the momentum amplitudes \(\phi (P_{1})\)  and \(\phi (P_{2})\) 
in nuclear wave functions as wave packets. Normally the total cross
section does not depend on the form of \(\phi (P_{1})\)  and \(\phi (P_{2})\) 
[1]}. A natural choice for \(\phi (P_{1})\)  and \(\phi (P_{2})\) 
is that they can be factorized as \(\phi (P_{i})\simeq \phi _{z}(P_{i}^{z})\phi _{T}(\mathbf {P}_{iT})\) 
with \(i=1,2\) , where \(\phi _{z}(P_{i}^{z})\)  are distributions of longitudinal
momenta that \(P_{1}^{z}\)  and \(P_{2}^{z}\)  are centered at \(P_{A1}^{z}\) 
and \(-P_{A1}^{z}\)  respectively, and \(\phi _{T}(\mathbf {P}_{iT})\) 
are distributions of transverse momenta that \(\mathbf {P}_{iT}\)  are
centered at zero. As a simple ansatz we assume the longitudinal part
takes the form
\(& & \phi _{z}(P_{1}^{z})\phi _{z}(P_{2}^{z})\phi _{z}^{*}(P_{1}^{\prime z})\phi _{z}^{*}(P_{2}^{\prime z})\nonumber \\& \approx & (2\pi )^{2}\delta \left[(P_{1}^{z}+P_{1}^{\prime z})/2-P_{A1}^{z}\right]\delta \left[(P_{2}^{z}+P_{2}^{\prime z})/2+P_{A1}^{z}\right]\nonumber \\& & \times G\left[\frac{1}{4}(P_{1}^{z}-P_{1}^{\prime z})^{2}\right]G\left[\frac{1}{4}(P_{2}^{z}-P_{2}^{\prime z})^{2}\right],\) 
